Who's talking about a day? The difference between a fast build tool and a slow tool is in the order of seconds, maybe minutes. Larger differences aren't down to the build tool but down to the codebase.
The difference between a tool that leverages incremental compilation effectively and one that almost always has to build from scratch (at least on CI) can be pretty huge and not "in the order of seconds".
That's more a CI issue than a build tool issue. Just about any (Java) build tool supports incremental compilation. But if your CI env doesn't have a place to cache those builds, it's not going to matter.
And even then it could be a matter of seconds on a smallish codebase. Nor everyone is building Google sized monorepos.
Even just doing things like "please predownload any dependencies in this one job and cache them, so they can be reused in these 5 other jobs" don't work reliably with maven because certain plugins like surefire download dependencies on the fly.
> Nor everyone is building Google sized monorepos.
Nor is everyone building "small-ish codebases". You don't have to be Google, I worked at a company with maybe 10 developers in total, but the codebase was over 10 years old and had a gazillion features (plus a lot of tech debt, obviously) and the build times were atrocious. It wasn't all maven's fault, of course (we had really badly written tests, for example), but the broken incremental compilation easily added 5-10 more minutes to each build.
I'm curious about those.